Formed fiberglass "liners" are sold here in New England at some landscape/garden centers which also carry garden pond accessories.Try any websites relating to garden ponds and you should find something that way.Another option is to get a roofing contractor to give you some scrap rubber roof material.Dig a hole,line and overlap the rubber outside the hole,weight rubber with rocks and fill'er up.

Thanks Matches...we'll give it a try.